Sex & Nudity

On The Last Stand, the show's finale, Korra and Asami plan to go on vacations to the Spirits World, just the two of them. On the ending, we see the two girls entering the Spirits World, holding hands and looking at each other. This moment might give us to understand that they are, at least by the end of the episode, a couple.

Spoilers

The Parents Guide items below may give away important plot points.

Sex & Nudity

In the series finale, two women enter a same-sex romantic relationship. Throughout the series, there are a lot of parallels to the original series' romantic relationships when it comes to these two women's relationship. In addition to that, there is a fair amount of subtext in both the vocal performances of the voice actresses and the writing itself.
It was later confirmed by the creators that they are bisexual.

Violence & Gore

Some characters can manipulate others by "blood-bending," causing them great harm this causes their movements to seem unnatural. This can be disturbing.
In one episode of Season 1, a father makes his two sons "blood-bend" a pack of wolves. In the same episode the father makes his sons "blood-bend" each other.

The main villain of Season 3 kills a significant character by suffocating said character with airbending. The scene is arguably the most graphic in the series, as the show does not cut away from the murder. At one point, the "camera" does a close-up of the suffocating characters face, showing her eyes red and bulging from oxygen depravation but does not show her dead on the floor.

One of the main villains of Season 3 is killed in an extremely violent way, when a metal plating is enclosed around her head, and an explosion goes off inside, blowing her head off. Though the scene cuts off at the very last second before the explosion, the implications about how the villain died are made very clear to the viewer.

One of the main villains of Season 3 is killed when a well-liked character electrifies the pond of water where the villain is standing on. The scene is mildly graphic, as it fully depicts the electrocution.

The main character of the show is poisoned,when poison is forcefully introduced in her body. The scene can be intense and frightening to younger children, as it depicts in detail not just the forceful poisoning, but also the main character's subsequent suffering as a result of the poison.